Rotomolding is used for the manufacture of simple to complex, hollow plastic products. It can be used to mold a variety of materials such as polyethylene, polypropylene, polycarbonate, polyamide, or polyvinyl chloride (PVC). In the rotomolding process, a polymer powder is placed inside a mold having the outer shape of the desired hollow product. The mold is constantly rotated in three-dimensions while being heated. The polymer powder adheres to the heated mold once the mold temperature is above the polymer melting point, producing a relatively uniform polymer layer on the entire inner surface of the mold, resulting in a hollow object the same shape as the mold. A multi-layer object can be produced by adding a second polymer to the mold after the mold is coated with the first layer.
It is often desirable to form a multi-layer rotationally molded article, to combine the attributes of two different polymeric materials.
Medium density polyethylene is preferably used in rotomolding processes, although crosslinked polyethylene may also be used. Polyethylene represents more than 80 percent of the polymers used in the rotomoulding market. This is due to the outstanding resistance of polyethylene to thermal degradation during processing, to its high resistance to alcohols, to its easy grinding, good flowability, and low temperature impact properties.
Polyamides are useful in rotomolded objects due to their high resistance to hydrocarbon permeability and excellent chemical resistance.
Polyamides used in rotomolding are generally of lower molecular weight and lower inherent viscosity. Conventional wisdom was that resin grades with molecular weights (Mw) higher than 20,000 and inherent viscosities higher than 1.05 dL/g are not normally perceived as being rotomoldable. The problem with the lower molecular weight and/or lower inherent viscosity polyamides is that the low temperature impact strength is not satisfactory for many end-use applications. For lateral impact testing of motorcycle tanks (for example, in SAE J1241), a pendulum weighing between 80 lb. and 160 lb. is released from a height sufficient to produce 450 Nm±10 Nm (4000 in-lb±100 in-lb) of kinetic energy at impact. Any tank damage resulting in external leakage in excess of 30 cc/min (1 fl. oz./min.) is considered a failure. Tanks made with lower molecular weight and/or lower inherent viscosity polyamides generally do not consistently pass this test.
One method to improve the low temperature impact strength of polyamides can be by the addition of one or more impact modifiers. However for some applications (e.g., fuel containment), the addition of impact modifiers can detrimentally affect certain properties (e.g., reduced resistance to fuel permeation).
There is need for a polyamide that can be used in a rotomolding process, which provides better low-temperature impact strength than currently used polyamides.
Surprisingly it has been found that high molecular weight and high viscosity polyamides can be successfully rotomolded into monolithic or multi-layer articles, and these articles have superior low temperature impact strength.